Raw crude oil generally contains salts, such as calcium, sodium and magnesium chlorides. Salts cause corrosion in refinery systems that are expensive to repair and require more frequent shutdown and longer turn-around before profitable operation resumes. Corrosion is caused primarily by hydrochloric acid (produced from the hydrolysis of salts at high temperatures) in crude oil distillation columns and overhead systems. Since salts in crude oils are a significant problem and concern, removing such salts is an important operational process in a refinery.
Typically, desalting crude oil involves adding water to the incoming crude oil emulsifying the water and oil by shearing across a globe valve, which is also known as a mix-valve and allowing the oil and water to separate in a desalter settling vessel. The salt preferentially and fairly rapidly dissolves into the water immediately following the mix-valve so the remaining step is to separate the water from the oil. The oil and water are separated based on their density differences. Desalted crude exits from the top of the desalter settling vessel to the crude distillation tower while effluent water or brine exits from the bottom. However, desalting heavy crude oil in a refinery desalter system is challenge due to relatively high viscosity of heavy crude and relatively high densities of heavy crude oil relative to the water that captures the salt and is then separated from the crude oil based on density differences. Moreover, water and oil emulsions for heavy crude oil tend to be more stable than for light oil and stable emulsions make desalting less successful or at least more difficult.
In a typical desalting process, raw crude oil containing salt is mixed with water and raises the water content to a range of about 3% to 10%. The mixing of the oil phase and the water phase is carried out using a single mix valve which creates the water and oil emulsion. With other process parameters remaining similar, the pressure drop across the mix valve determines the size of the water droplets in the emulsion. Poor mixing across the mix valve allows salt to carry-over with the crude and over-mixing results in the formation of a stable emulsion which is difficult to break in the refinery desalter.
Within the desalter settling vessel, water droplets undergo coalescence under the influence of electrical and gravitational fields. In a traditional desalter, large water droplets settle down to the bottom of the desalter tank whereas smaller drops have a low settling velocity and tend to become entrained with the crude oil and exit the desalter into a stream that is a hazard to refinery systems as described above. The size of the droplets which actually settle downward within the desalter can be estimated based on the centerline velocity of the crude oil.
For those that have studied and designed desalting systems, desalting efficiency is generally defined in Equation (1) as:Desalting Efficiency (%)=(Salt In−Salt Out)/Salt In×100  Equation (1)
Desalting efficiency may be described as the product of the mixing efficiency and the dehydration efficiency. “Salt In” may be described as the salt content of the incoming oil, and “Salt Out” may be described as the salt content of the exiting oil. The mixing efficiency, while not commonly measured, is the percentage of salt transferred to the bulk water phase. The dehydration efficiency is described in Equation (2) as:Dehydration Efficiency (%)=(Water In−Water Out)/Water In×100  Equation (2)
“Water In” is the combined contribution of added water and the inlet percent of basic sediments and water (“% BS&W”) in raw crude oil. “Water Out” may be described as the percent of basic sediments and water (“% BS&W”) in desalted crude oil.
Due to constantly changing operating conditions, operation of the mixing valve is often constrained by the rate of water separation in desalter vessel. Excessive pressure drop across mixing valve promotes mixing and salt transfer, but such intense mixing creates an emulsion with relatively smaller average water drop size. Such an emulsion is difficult to break and separate and it lowers the dehydration efficiency.
Dehydration (water separation) in a desalter depends on the net velocity (“UNet”) of water drops is given in Equation (3) as:UNet=(r2×(ρw−ρo)×g/3μo)−(Qo/(L×D))  Equation (3)
The second term on the right-hand side “QO/(L×D)” is the settling velocity of a water drop of radius “r”, viscosity “μo,” and density “ρw” in oil of density “ρo” and viscosity “μo.” The pre-factor of one-third (⅓) is appropriate for a viscous water drop as opposed to a rigid spherical particle. The second term is (approximately) the centerline velocity arising from the upward oil flow (Qo) in the desalter of diameter D and length L. Simply put, factors that increase the net positive (i.e. downward) velocity of the water drops improve dehydration in a desalter.
The ratio of the water oil density differential to oil phase viscosity ((ρw−ρo)/μo), which is referred to as Stokes' parameter, depends on the water, crude oil, and operating temperature, whereas the drop size, r, is a function of the shear rate and flow geometry at the mixing valve (and subsequent pipe and fittings) and the water-oil interfacial properties.
There is a difficult trade-off between high shear which captures more salt in the water but allows more of the water to go into the refinery and low shear which prevents water from passing along into the refinery, but captures less of the salt in the crude. Adding to this challenge, the viscosity of the heavier crude oils tends to slow the settling velocity of all water droplets. The density difference between water and heavy crude oil is less than lighter crude oils further slowing settling velocity. Thus, as the worlds' production of crude oils tends to get heavier and denser, refineries will need to deal with the challenges within the desalters.
What is needed then are improved methods, processes and apparatuses to improve desalting of crude oil in an oil refinery.